In a conventional arrangement, a plurality of magnetic discs of the type included in data processing memories are coaxially stacked on the same shaft to form a high-capacity memory unit which functions by means of a "comb." Each tooth of the comb carries at least one read/write head under which one face of one of the discs revolves. Radial movement of the read/write heads provides direct access to all the concentric recording tracks in one stack of discs. In this way, it is possible simultaneously to read or write a total number of bits which is twice the number of discs in the stack. Consequently, this arrangement is widely used in systems requiring a large processing capacity.
The contents of such a memory unit can only be altered by erasing the recorded data and then recording fresh data at the same point. This does not present any appreciable problem in large systems equipped with auxiliary memories, such as tape memories. However, in systems of smaller size which perform various data-processing operations selectively but repeatedly, for example to control a machine tool or to perform/accounting operations, it is desirable to alter the contents of a disc memory without losing data. For this purpose it is known to use magnetic discs provided with individual cassettes, the shape and size of which are standardized to enable the cassettes to be interchanged. Cassettes of this kind previously proposed have numerous drawbacks. In particular, (1) complicated operations are required to make them operable, (2) they do not provide complete protection for the disc when not in use, that is to say between two periods of operation, and (3) they hamper cooling and proper cleaning.
Consequently, an object of the invention is to provide an improved magnetic disc cassette which is of a suitable nature to remove the disadvantages mentioned of known designs.